io8 MODERN PAPER-MAKING used by all up-to-date mills. Among pulpers one of the most popular is the Perfect Pulper (Fig. 33). This machine consists of a cast-iron shell of conical shape, within which is a revolving shaft, on which is fitted a series of curved iron teeth or arms. The inside of the conical shell is also studded with shorter teeth, between which the revolving arms turn. At the smaller end of the cone is a hopper into which the broke is fed together with a supply of hot water for hard papers or cold for softer stuff. At this end the teeth are short and stout, gradually increasing in length to fill the cone, so that the pulp travels towards the larger end as it is broken up, the longer ones having the more disintegrated pulp to work through. The revolving shaft and teeth have a speed of 50 to 60 revolutions per minute and form a sort of screw, which gradually draws the stuff to the discharge opening. The conical casting [Watford Engineering Works FIG. 34.—-WATFORD PATENT PULPER is in two parts, of which the top half is hinged for obtaining access to the interior. The action is continuous, but may be made intermittent if desired by closing the discharge doors. The output is from 6 to 20 cwt. per hour, according to the size of the pulper and the quality of the broke. The power required is from 15 to 36 h.p. Hot or cold water or steam may be used while feeding in the stock, but only soft papers can be properly pulped without heat. No colour, size, loading or fine fibres are lost by this method of pulping. Another very popular machine of the same type is the Watford pulper. Some pulpers are still in use which work with intermittent charges. These are similar in construction to the dough mixers which are used in bakeries. They consist of metal troughs in which revolve two curved arms with serrated edges. The trough is gradually filled with the stock and a supply of hot water. The broke is usually disintegrated by the time the pulper is folly loaded, which